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Explanation: They mean to stretch the muscles in those body areas. For example the muscles of the buttock(s) (the middle and greatest gluteal muscles or gluteus) and the many muscles on the front and rear of the thigh(s) (rectus femoris, biceps femoris, vastus lateralis, semitendinous, etc, etc, etc) are all shown in Reuter & Reuter Medizinisches Wörterbuch.
I think I would just write "hip and thigh muscles" because your text does not sound too technical. Your author seems to want to get away from terms like "Hintern" or "Po", so I would not use "fanny", "backside", "bottom" etc.